1. Field of the Invention
The present invention relates to a CMOS image sensor, and more particularly to a CMOS image sensor that controls a reset voltage to reduce reset noise caused by a reset operation of the CMOS image sensor, fixed pattern noise caused by different characteristics of detection circuits, and image lag caused by the influence of a previous image signal upon the current output signal, thereby achieving a high signal to noise ratio.
2. Description of the Related Art
A CMOS image sensor uses PN-junction photodiodes, which can be formed by a CMOS manufacturing process, to convert the intensity of incident light to an electrical signal. One of the most important requirements of the CMOS image sensor is a high sensitivity to the incident light.
As well known in the art, unlike the CMOS image sensor, a conventional Charge Coupled Device (CCD) image sensor detects an optical signal through charge coupling, instead of transistor switching. A photodiode, which is provided for each pixel, serves to detect light incident on the pixel, does not output an optical detection current immediately after the detection, but instead outputs the detection current after accumulating it for a certain accumulation time, so that the detection signal voltage can be raised proportional to the certain accumulation time, thereby increasing light sensitivity and decreasing noise.
FIG. 1 is a circuit diagram illustrating the configuration of each pixel circuit in a general CMOS image sensor.
A reset operation of the CMOS image sensor is a process in which a reset switch 10 is turned on to remove a photo-induced charge accumulated on a photodiode PD.
After the reset operation, a new photo-induced charge according to incident light is accumulated for a certain time, and then a selection switch 12 is turned on so that a voltage determined based on the accumulated photo-induced charge is read out through an amplifier 11 and the selection switch 12.
Three main types of noise occurring during the operation of the CMOS image sensor are reset noise, image lag noise, and fixed pattern noise. The reset noise is caused when the initial voltage becomes unstable or uncertain due to a photo-induced charge left unstably or uncertainly in the photodiode since the activation of the reset switch has failed to completely remove the photo-induced charge from the photodiode. The image lag is a phenomenon in which a residual image of the previous pixel image is left after a reset operation is completed since part of a photo-induced charge accumulated until the reset operation is started is left after the reset operation is completed. The fixed pattern noise is caused by different detection voltages of the pixels due to different characteristics (typically, different threshold voltages) of amplifier transistors when the photo-induced charge accumulated in each pixel is detected through the amplifier transistor provided in each pixel.
On the other hand, general image capturing and processing devices employ an analog-to-digital converter for converting an analog output voltage of each pixel to a digital value.
FIG. 2 is a circuit diagram showing a general CMOS image sensor that includes an analog-to-digital converter in each column of pixels.
As shown in FIG. 2, the CMOS image sensor includes an array of pixels 8, each of which includes a photodiode and an amplifier, a row controller 2 and a column controller 4. In addition, an analog-to-digital converter 6 is provided for each column of pixels to convert an analog voltage output of a pixel 8 in the column and in a row selected by the row controller 2 to a digital value. An optical detection signal of a pixel 8 in a column selected by the column controller 4 is output after being converted to a digital image signal through an analog-to-digital converter 6 corresponding to the selected column.
When compared to a CMOS image sensor including a single analog-to-digital converter for signal conversion of all pixels, the CMOS image sensor of FIG. 2 configured as described above is advantageous in that the analog-to-digital converter is easy to design and the conversion speed is high.
However, since the columns provide different pixel outputs under the same conditions due to different characteristics of the analog-to-digital converters 6, there is a need to provide a method for compensating for the pixel output difference.
One conventional compensation technique employs Correlated Double Sampling (CDS) that uses a pin diode to reduce the reset noise as well known in the art. However, this technique requires an additional design and manufacturing process for forming the pin diode and also requires a high driving voltage.
Another compensation technique employs a circuit for controlling a reset voltage in a general photodiode-based pixel to reduce the reset noise. An example of this technique is “active reset readout” proposed by Fowler et al (U.S. Pat. No. 6,424,375), which controls a reset voltage through feedback amplification in each pixel to reduce the reset voltage.
B. Pain et al have also proposed a method for attenuating the noise voltage through the feedback operation in a paper “Reset noise suppression in two-dimensional CMOS photodiode pixels through column-based feedback-reset” in 2002 IEEE International Electron Devices Meeting.
The conventional methods described above may be effective in reducing the reset noise and the image lag. That is, since the conventional methods do not perform the compensation after the analog-to-digital conversion, the conventional methods reduce the reset noise or noise factors such as the image lag, but they require a decrease by at least a threshold voltage in the final reset voltage, as compared to the conventional reset voltage, thereby reducing the range of output signals, and also cannot simultaneously suppress the fixed pattern noise that may subsequently occur in the analog-to-digital converter, etc.